Description

As per our Wednesday meeting, this adds a parse error when something that would be parsed as an external call is present at the top level, unless the head of the external call begins with a caret (to make it explicit).

I tried to make the error quite descriptive about what should be done.

User-Facing Changes

These now cause a parse error:

$foo = bar
$foo = `bar`

These would have been interpreted as strings before this version, but now they'd be interpreted as external calls. This behavior is consistent with let/mut (which is unaffected by this change).

Here is an example of the error:

Error:   × External command calls must be explicit in assignments
   ╭─[entry #3:1:8]
 1 │ $foo = bar
   ·        ─┬─
   ·         ╰── add a caret (^) before the command name if you intended to run and capture its output
   ╰────
  help: the parsing of assignments was changed in 0.97.0, and this would have previously been treated as a string.
        Alternatively, quote the string with single or double quotes to avoid it being interpreted as a command name. This
        restriction may be removed in a future release.

Tests + Formatting

Tests added to cover the change. Note made about it being temporary.